In a magnetic recording device in which a read head is based on a tunneling magnetoresistive (TMR) sensor or a giant magnetoresistance (GMR) sensor, there is a constant drive to increase recording density. One method of accomplishing this objective is to decrease the size of the sensor element in the read head that is suspended over a magnetic disk on an air bearing surface (ABS). The sensor is a critical component in which different magnetic states are detected by passing a sense current there through and monitoring a resistance change. In a GMR configuration, two ferromagnetic layers are separated by a non-magnetic conductive layer in the sensor stack while in a TMR sensor, a tunnel barrier layer such as AlXOY separates the two ferromagnetic layers. One of the ferromagnetic layers is a pinned layer wherein the magnetization direction is fixed by exchange coupling with an adjacent anti-ferromagnetic (AFM) pinning layer. The second ferromagnetic layer is a free layer wherein the magnetization vector can rotate in response to external magnetic fields. In the absence of an external magnetic field, the magnetization direction of the free layer is aligned perpendicular to that of the pinned layer by the influence of hard bias layers on opposite sides of the sensor stack. When an external magnetic field is applied by passing the sensor over a recording medium on the ABS, the free layer magnetic moment may rotate to a direction which is parallel to that of the pinned layer.
A sense current is used to detect a resistance value which is lower when the magnetic moments of the free layer and pinned layer are in a parallel state. In a CPP configuration, a sense current is passed through the sensor in a direction perpendicular to the layers in the sensor stack. Alternatively, there is a current-in-plane (CIP) configuration where the sense current passes through the sensor in a direction parallel to the planes of the layers in the sensor stack.
Ultra-high density (over 100 Gb/in2) recording requires a highly sensitive read head in which the cross-sectional area of the sensor is typically smaller than 0.1×0.1 microns at the ABS plane. Current recording head applications are typically based on an abutting junction (ABJ) configuration in which a hard bias layer is formed adjacent to each side of a free layer in a GMR spin valve structure. As the recording density further increases and track width decreases, the junction edge stability becomes more important so that edge domain formations in the free layer are prevented. In other words, longitudinal biasing is necessary so that a single domain magnetization state in the free layer will be stable against all reasonable perturbations while the sensor maintains relatively high signal sensitivity.
In longitudinal biasing read head design, films of high coercivity material are abutted against the edges of the GMR sensor and particularly against the sides of the free layer. By arranging for the flux flow in the free layer to be equal to the flux flow in the adjoining hard bias layer, the demagnetizing field at the junction edges of the aforementioned layers vanishes because of the absence of magnetic poles at the junction. As the critical dimensions for sensor elements become smaller with higher recording density requirements, the minimum longitudinal bias field necessary for free layer domain stabilization increases.
A high coercivity in the in-plane direction is needed in the hard bias layer to provide a stable longitudinal bias that maintains a single domain state in the free layer and thereby avoids undesirable Barkhausen noise. This condition is realized when there is a sufficient in-plane remnant magnetization (Mr) which may also be expressed as Mrt since Mr is dependent on the thickness (t) of the hard bias layer. Mrt is the component that provides the longitudinal bias flux to the free layer and must be high enough to assure a single magnetic domain in the free layer but not so high as to prevent the magnetic field in the free layer from rotating under the influence of a reasonably sized external magnetic field.
Referring to FIG. 1, a conventional abutted junction hard bias (ABJ-HB) structure in a read head 1 with a GMR sensor is shown. The substrate 2 may be comprised of a first gap layer on a first shield layer (not shown). Note that the sensor element generally has sloped sidewalls wherein the top surface 3b is narrower than the bottom surface 3a. Moreover, the sensor element 3 may be a bottom spin valve, a top spin valve, or a multilayer spin valve. In a typical bottom spin valve configuration (not shown), a seed layer, AFM layer, pinned layer, spacer, free layer, and a cap layer are successively formed on the substrate. A top spin valve (not shown) generally has a seed layer, free layer, spacer, pinned layer, AFM layer, and cap layer successively formed on the substrate.
The ABJ-HB structure consists of a seed layer 4 formed on the substrate 2 and along each side of the sensor element 3, and the overlying hard bias layers 5 that have a proper microstructure due to the crystalline nature of the seed layers. The hard bias layers 5 form an abutting junction on either side of the free layer (not shown) in the sensor element 3. Leads 6 are provided on the hard bias layers 5 to carry current to and from the sensor element 3. The distance between the leads 6 defines the track width TW of the read head 1. Above the leads 6 and sensor element 3 are successively formed a second gap layer 7 and a second shield layer 8.
The conventional ABJ-HB design has been employed for magnetic sensor stabilization for several production generations. However, with further reduction of the magnetic read width (MRWu) to less than 0.3 microns, the ABJ-HB configuration tends to fail in producing sufficient biasing efficiency. In other words, the conventional ABJ-HB structure either reduces the sensor amplitude too much or causes a loss in sensor stability when the hard bias layer is either too thick or too thin. Moreover, the dead zone which is the area in the sensor element between the MRWu and the track width is always large. Therefore, further optimization in hard bias materials or in junction geometry is necessary to achieve high performance magnetic read heads that satisfy the newest design requirements.
In related art, an additional AFM layer is provided between a hard bias layer and an overlying lead in U.S. Pat. No. 6,779,248 so that there is no fall off in bias strength before the edge of the gap is reached. In U.S. Pat. No. 6,760,966, a soft magnetic layer is added above hard bias layers to provide flux closure to the hard bias layers and thereby prevent flux leakage into the gap region.
A magnetoresistive effect head with an improved output is described in U.S. Pat. No. 6,545,847 and includes a design wherein the hard bias structure is comprised of a stacked layer of an alloy of NiFe and an AFM film such as FeMn, NiMn, or CrMn. The hard bias structure is disposed on a seed layer and below an electrode.
In U.S. Pat. No. 5,754,376, a longitudinal bias applied to a soft magnetic layer is weak enough that the sense current flowing through the magnetoresistive (MR) conductive layer sufficiently magnetizes a SAL transversely.
U.S. Pat. No. 6,469,878 discloses exchange tabs which are formed above and adjacent outer portions of a free layer to bias the free layer. The exchange tabs are formed from the same AFM material as in the first pinning layer.
In U.S. Patent Application 2004/0105192, poor squareness and coercivity resulting from lattice distortion in a seed layer that contacts an AFM layer in a bottom spin valve is corrected by inserting a NiCr, NiFe, or Cr film between the seed layer and AFM layer thereby producing a smoother surface onto which the longitudinal bias structure is deposited.
A longitudinal bias layer comprised of an AFM layer formed over a ferromagnetic layer of Fe, Co, Ni, or NiFe which is disposed on either side of a protective film on a free layer is described in U.S. Pat. No. 6,338,899. The longitudinal bias structure may also be a laminate that includes an underlayer/FM layer/AFM layer configuration.
In U.S. Patent Application 2002/0191354, a sidewall layer is formed on a magneto-resistive element by oxidizing, nitrifying, fluoridating, carbonizing, sulfurating, or boronizing the side surface of the MR element. The specular reflecting effect is increased while the sidewall layer maintains the biasing effect of an adjacent hard bias structure.